Birds and bees and syringes: Advancements in equine reproductive technology

Picture it–a dark desert night 700 years ago. An Arab chief creeps into a rival's camp and steals away with the prize he sought—semen from the rival's stallion, which he uses to artificially inseminate his own mare.

That was the first recorded use of artificial insemination for horse breeding, in 1322. Unverified accounts report equine AI dating back to before the birth of Christ, according to "Artificial Insemination in the Horse," by J.M. Bowen.

The first experiments with cooling semen happened about the time the United States was becoming a country, in 1776. Further success was documented in the 1880s, when it was used to overcome infertility in mares.

Today, artificial insemination is a common practice among breeders and technology and research have improved the conception rates and made it—in many cases—more economical and practical than natural service.

Dr. Al Flint, a partner in Montana Equine, said one of the most important advances in AI in recent years is the improvement of the process and products used for freezing semen. Recent developments in semen extenders, which prevent the sperm cells from bursting when frozen, improve the results with frozen semen.

Using frozen semen allows the breeder flexibility, giving them more time to use the semen, as opposed to cooled semen, which must be used within 48-72 hours after collection, Flint estimates.

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The freezing process also allows semen from stallions to be saved for future use, even after his death in some cases.

A drawback is mares must be inseminated with frozen semen shortly after ovulation, a four- to six-hour window, instead of the 12-hour window with cooled semen, which can be used before ovulation.

"The biggest drawback of frozen semen is the team it takes to utilize it," said Flint. "You have to have a vet or reproductive professional who can basically monitor these mares 24 hours a day."

That adds to the cost, though Flint estimates that in his clinic, the difference between using cooled and frozen semen is between $100-125.

Historically, success rates have been better with cooled semen than with frozen semen, but Flint says with proper management, the gap in success rates is closing between the two methods.

Embryo transfer (ET) is another technology that has changed the breeding world, and the performance world for some mares.

"More people are looking at embryo transfer as a viable option for managing problem mares, or mares that are in performance work," Flint said.

Harvesting embryos allows a mare to stick to a rigorous performance schedule while a recipient mare gestates and raises her foal.

"Some people take a year off, or give us four to six weeks to collect as many embryos as we can," Flint said.

The first foal born as a result of ET was in 1974 in Japan, the same time as the first successful attempt at superovulation in Wisconsin.

Superovulation is the use of hormones to prompt mares to release more oocytes (eggs).

"We get asked all the time about superovulation of mares," Flint said. "It's expensive and it doesn't increase the rate of ovulation by much."

He said unlike cattle, in which superovulation produces about 10 times more embryos than natural ovulation, that rate goes from 1.4 oocytes per natural ovulation to 1.8 with superovulation in horses.

That doesn't sound like much compared to the prolific nature of cattle, but it can mean a valuable mare could produce four to five times as many offspring across the span of her life, all while still working in the performance ring.

Flint said the ideal time to harvest embryos is when the mare is 6 to 10 years old, although embryos can be collected from mares as young as 4. The number of viable embryos diminishes with age, but Flint says, "The simple answer is that no, a mare is never too old."

Flint says they get a viable embryo about 60 percent of time they attempt to collect, and about 75 to 80 percent of those turn into successful pregnancies. Mares typically ovulate from mid-February through the end of August, cycling every 17-21 days. That gives them an opportunity to collect eight or nine times. Given the averages, that will result in about four live foals per year. "A good average would be about four to five embryos per year," Flint said.

Managing a mare intensely for reproduction generally has no negative effect on her health, Flint said. However, it's best to occasionally let nature take its course. "Mares that have had multiple seasons of ET should be allowed to carry a foal to 'reset' the system for their reproductive health," Flint said. "You can do ET for a couple of years, then allow the mare to carry a foal, then you can go back to ET. You can do that for as long as she's reproductively healthy."

As always, research is continuing to improve the processes and to develop new tools. Sexing semen is one of those technologies that has been successful, but isn't perfect yet. Flint said this research is progressing slower than it does in cattle because equine semen is more fragile than bovine semen, so it is difficult to sort it and maintain its viability.

Another area of research right now is oocyte transfer, which would allow for the collection of oocytes—eggs that aren't fertilized– in mares that can't get pregnant or maintain a pregnancy.

Freezing oocytes would allow for the collection of oocytes from a mare that has a terminal illness or a critical injury, then those oocytes could be used in the future, rather than having to be fertilized and put in recipient mares right away.

In the last 700 years reproductive technology has come a long way, but so far, it still takes a good mare and a good stallion to produce a good foal.